At present, network applications gradually develop towards multimedia services. As an important component of multimedia service, video has become one of main carriers about information transmission. New requirements for video quality have been put forward by wide application of video. Resolution of each kind of video at present becomes larger. Meanwhile, growth rate of video data size exceeds development rate of storage device and network bandwidth. Thus, long-term and widespread attention has been paid to video compressing and encoding technologies.
In existed video compression and encoding technologies, video is generally divided into I frame, P frame and B frame to be respectively encoded. I frame is an intra-frame encoding frame, which may only be predicted with intra-frame information during encoding process. In a decoding process, I frame image may be decoded and reconstructed with code stream of current frame. Encoding and decoding processes of I frame are independent, which are not relevant with adjacent frames, e.g., front and rear frames. In the encoding processes about P frame and B frame, in addition to adopting intra-frame prediction technologies, inter-frame prediction may also be executed. In the inter-frame prediction technologies, it is necessary to search in a reference frame for a block most similar to current Macro Block (MB), to be predicted. While, in the decoding process, a prediction block may be found from the decoded reference frame, according to motion vector information, so as to implement decode. The encoding and decoding processes about P frame and B frame are not independent, which have high time complexity, utilize inter-frame time correlation and intra-frame space correlation, so as to implement efficient compression. Compared with the encoding process of P frame and B frame, I frame may only utilize the intra-frame space correlation to execute prediction, the encoding process thereof is independent and has lower complexity. However, compression efficiency of I frame is far from that of P frame and B frame.
At present, the intra-frame prediction is mainly applied in the encoding process about I frame. However, in the P frame or B frame, when it is difficult to search out accurate prediction block with inter-frame motion search technologies, the intra-frame prediction may also be adopted by few MBs.
At present, in Moving Picture Exports Group-2 (MPEG-2) standards, a simple airspace prediction algorithm may be adopted by the intra-frame prediction. Each pixel in an encoding block may use pixel value 128 to execute prediction, instead of utilizing information of the encoding frame to execute the prediction. For most of the sequences, prediction residual of the method is larger. Thus, compression efficiency of I frame which adopts such intra-frame prediction method is lower. In MPEG-4 standards, an intra-frame encoding block adopts Direct Current (DC)/Alternating Current (AC) prediction algorithm of frequency domain. Firstly, the encoding block executes a Discrete Cosine Transform (DCT) to the frequency domain, performs prediction on coefficient of current block utilizing DC/AC coefficient of an adjacent block. By using the MPEG-4 method, compression efficiency of I frame is improved. In H.264/Advanced Video Coding (AVC), an intra-frame prediction algorithm of direction prediction is adopted. Regarding each pixel block of 16*16, 8*8, or 4*4, the algorithm may execute the prediction from a certain direction by utilizing upper, left, upper left, upper right encoded blocks thereof. The compression efficiency of I frame may be greatly improved by using the direction prediction technologies. However, in the prediction process in which block is taken as a prediction unit, only correlation of pixels among blocks is utilized. However, correlation between adjacent pixels within a block is not fully utilized. For an entire block, it is difficult to accurately predict a pixel far from a prediction pixel within a block. Thus, prediction residual within a frame is larger. Particularly to a block with complicated texture information, it is difficult to achieve good prediction, which directly leads to lower compression efficiency of intra-frame encoding. Subsequently, compression efficiency of whole video sequence may be affected.
However, for video images with same contents, correlation between pixels is larger accompanying with higher resolution. However, such space correlation is not fully utilized in existed intra-frame encoding technologies. To address this shortcoming, it is of great significance to study a prediction algorithm which fully utilizes correlation among pixels, so as to improve compression efficiency of video encoding.